A filter is a device for passing (filtering) signals of only a certain frequency band from among the inputted frequency signals, and is implemented in various ways. The band-pass frequency of an RF (radio frequency) filter may be determined by the inductance and capacitance components of the filter, and the operation of adjusting the band-pass characteristics of a filter is referred to as tuning.
Certain frequency bands may be allotted to businesses dealing with communication systems, such as mobile communication systems, where such communication businesses may divide the allotted frequency bands into several channels for use. In the related art, communication businesses generally manufactured and used a separate filter that is for suitable for each frequency band.
In recent times, however, rapid changes in the communication environment have created a need for a filter to have variable properties, such as for the center frequency and bandwidth, for example, unlike the earlier environment for mounting filters. For varying the properties in this manner, a tunable filter may be used.
FIG. 1 illustrates the structure of a tunable filter according to the related art.
Referring to FIG. 1, a filter according to the related art may include a housing 110, an input connector 120, an output connector 130, a cover 140, and multiple numbers of cavities 150 and resonators 160.
A number of walls may be formed within the housing 110, with the walls defining cavities 150 in which to hold the resonators, respectively. The cover 140 may include tuning bolts 170, as well as joining holes for joining the housing 110 with the cover 140.
The tuning bolts 170 may be coupled to the cover 140 and may penetrate inside the housing. The tuning bolts 170 may be arranged on the cover 140 in corresponding positions in relation to the resonators or in relation to particular positions inside the cavities.
RF signals (or frequency signals) may be inputted by way of the input connector 120 and outputted by way of the output connector 130, where the RF signals may progress to the next cavity 150 through the coupling window formed in each cavity 150. Each of the cavities 150 and resonators 160 may generate a resonance effect of the RF signals, so that the RF signals may thus be filtered by this resonance effect.
In a filter according to the related art, such as that shown in FIG. 1, the tuning of frequency characteristics such as center frequency and bandwidth may be achieved using the tuning bolts 170.
FIG. 2 is a cross-sectional view of a cavity in a filter according to the related art.
Referring to FIG. 2, a tuning bolt 170 may penetrate through the cover 140 to be located above a resonator 160. The tuning bolt 170 may be made of a metallic material and may be secured to the cover 140 by way of screw-joining.
Hence, the tuning bolt 170 can be rotated to adjust its distance to the resonator 160, and by thus varying the distance between the resonator 160 and the tuning bolt 170, tuning may be achieved. The tuning bolt 170 can be rotated manually, or a separate machine can be employed for rotating the tuning bolt. If the tuning is achieved at an appropriate position, the tuning bolt 170 may be secured by a nut.
In a filter according to the related art, rotating the tuning bolt 170 to vary the distance between tuning bolt 170 and the resonator 160 also causes the capacitance to vary. Capacitance is one of the parameters that determine the frequency of a filter, and therefore the center frequency of a filter can be changed by altering the capacitance.
With such a filter according to the related art, tuning is possible only at the initial fabrication stage, and its structure makes it difficult to accomplish tuning during use. In order to solve such difficulties, a tunable filter was proposed which employs a sliding system, with which tuning can be performed more easily.
For a tunable filter using a sliding system, a sliding member capable of sliding is installed between the cover 140 and the resonators 160, and tuning elements made of metallic or dielectric material are attached to a lower portion of the sliding member, after which the frequency band characteristics of the filter, such as resonance frequency and bandwidth, may be tuned by the sliding motion of the sliding member. The sliding member can be made to slide automatically using a motor, or can also be made to slide manually by a user.
Such a tunable filter using a sliding system has the advantage of enabling tuning just by moving the sliding member left and right.
With a tunable filter using a sliding member, however, there is the problem that, in order to obtain the band-pass characteristics desired by the user, each and every motor has to be rotated while checking whether or not the desired band-pass characteristics are provided. In particular, if the tunable filter is installed in a region such as a mountainous area that is not easy to access, there is difficulty involved in having to actually reach the location where the tunable filter is installed when tuning the band-pass characteristics.